Metal-ceramic substrates and especially also copper-ceramic substrates are known in the most varied versions and are used in particular for electrical or electronic circuits and thus especially for power circuits (power modules).
In the simplest case these substrates consist of a single ceramic layer which is provided on both surface sides with metallic coating or a metal layer and thus preferably with a copper layer, the metal layer on the one surface side of the ceramic layer being preferably configured, i.e., forming printed conductors and/or contact surfaces, etc., of which during operation of the electrical circuit at least one part has a voltage potential which is different from the voltage potential of the other metal layer on the other surface side of the ceramic layer. By means of the ceramic layer fundamentally a relatively high dielectric strength between the metal layers on the surface sides of the ceramic layer is ensured. However, often these metal-ceramic substrates in certain areas are also often formed such that the metal layers on the two surface sides of the ceramic layer, for example, on the edge of the ceramic layer or in the area of the recesses, openings, interruptions, etc. provided in this ceramic layer are electrically separated from one another only on an air path along the edge of the ceramic layer so that sufficient dielectric strength or sufficient insulation distance between the metal layers cannot be achieved there on the two surface sides of the ceramic layer for higher voltages. Furthermore, cracking occurs preferably at these points, especially when the substrates are subjected to repeated temperature changes. Cracking is caused by the corresponding mechanical stresses which are the result of different coefficients of expansion.
Generally, in these cases the dielectric strength is improved by using insulating compounds. In existing metal-ceramic substrates, dielectric strengths cannot be reached which are sufficient for higher voltages, i.e. for voltages, for example, higher than 1000 V. Occurrence of thermomechanically induced cracks cannot be prevented in this way.
It addition, it is also known that ceramic-metal substrates are produced by attaching (U.S. Pat. No. 4,500,029) metal foils or metal boards flat for forming metal layers on the surface side of the ceramic layer by means of the so-called direct bonding process (DB process) or eutectic bonding process, in which it is also known in these substrates that a metal layer is formed on one surface side of the ceramic layer in at least one sub-area in the manner of a bridge, such that this metal layer has a distance from the plane of the pertinent surface side of the ceramic layer there. A printed conductor provided on the same surface side of the ceramic layer is routed through the bridge-like section or area in this known configuration of a metal-ceramic substrate.
In addition, attaching a metal strip on the top of a ceramic layer by means of the DB method so that this strip projects with a sub-area above the border of the ceramic layer is also known (DE 25 08 224). The metal strip is first oxidized on one surface side for this purpose. The oxide is then removed on the sub-area with which the metal strip will project above the edge of the ceramic layer. Then the metal strip with its remaining oxidized surface side is placed on the ceramic layer and then by heating the ceramic layer and metal strip to a process temperature which exceeds the eutectic temperature of the metal oxide, but which is below the melting point of the metal of the metal strip, the bond is produced.